1. Field of the Invention
The present invention generally relates to the field of medical implants and more particularly to instruments used to insert spinal implants. An embodiment of the invention relates to instruments that may be used to form a spinal implant in a disc space between adjacent vertebrae during a spinal fixation procedure.
2. Description of Related Art
An intervertebral disc may be subject to degeneration. Degeneration may be caused by trauma, disease, and/or aging. An intervertebral disc that becomes degenerated may have to be partially or fully removed from a spinal column. Partial or full removal of an intervertebral disc may destabilize the spinal column resulting in subsidence or deformation of vertebrae. Destabilization of a spinal column may result in alteration of a natural separation distance between adjacent vertebrae. Maintaining the natural separation between vertebrae helps to prevent pressure from being applied to nerves that pass between vertebral bodies. Excessive pressure applied to the nerves may cause pain and/or nerve damage. During a spinal fixation procedure, a spinal implant may be inserted within a space created by the removal or partial removal of an intervertebral disc between adjacent vertebrae. The spinal implant may maintain the height of the spine and restore stability to the spine. The spinal implant may be a fusion device. Intervertebral bone growth may fuse the implant to adjacent vertebrae. The spinal implant may be an artificial disc.
A spinal implant may be inserted during a spinal fixation procedure using an anterior, lateral, or posterior spinal approach. An anterior spinal approach may be a preferred method for some spinal implant procedures. An anterior spinal approach may require less bone removal and muscle distraction than a posterior spinal approach. In addition, an anterior spinal approach may involve less risk of nerve damage than a posterior spinal approach.
During an anterior spinal approach, a surgical opening may be made in the abdomen of a patient. This opening may extend from the abdomen to an anterior surface of the spine. For some patients, the opening may be ten or more inches in depth. The opening needs to be large enough to accommodate instrumentation for insertion of a spinal implant within a disc space. A discectomy may be performed to remove or partially remove a defective or damaged intervertebral disc. The discectomy creates a disc space for a spinal implant. The amount of removed disc material may correspond to the size and type of a spinal implant to be inserted.
A type of spinal implant used to promote fusion of adjacent vertebrae may include a pair of engaging plates and struts. Struts may be positioned between the engaging plates to establish a separation distance between the engaging plates. Spinal implants having engaging plates and struts are described in U.S. Pat. No. 6,045,579 issued to Hochschuler et al., which is incorporated by reference as if fully set forth herein. Struts may separate and join engaging plates of an assembled spinal implant. The engaging plates may provide a large contact area between the spinal implant and vertebrae that are to be fused together. The large contact area may minimize subsidence and deformation of the vertebrae during use. The engaging plates may include protrusions that inhibit migration of the inserted implant. Each engaging plate may also include several openings to promote bone growth through the spinal implant to fuse the adjacent vertebrae together. The spinal implant may be formed in the disc space so that no over-distraction of the adjacent vertebrae is needed.
The engaging plates and struts may allow an anterior height of a spinal implant to differ from a posterior height of the implant. The availability of spinal implants that have different anterior and posterior heights may allow a surgeon to choose a spinal implant that will provide proper lordotic alignment and vertebral separation for a particular patient.
The struts of a spinal implant establish a separation distance between the engaging plates of the implant. The separation distance between the engaging plates may in turn establish a desired separation distance between adjacent vertebrae when the implant is formed between the vertebrae. Establishing a desired separation distance between engaging plates and adjacent vertebrae may establish a proper distance between vertebral bodies.
The struts of a spinal implant may include load-sharing members. The load-sharing members may allow a portion of a load placed on the implant to be transmitted to bone growth material placed within the implant. The transmittal of a portion of a load to bone growth material may promote bone growth in accordance with Wolff's law. Bone growth through and around a spinal implant may fuse the adjacent vertebrae together. Bone growth material inserted into the implant may be, but is not limited to, autograft bone harvested from a secondary location, such as the iliac crest; allograft material; or artificial bone growth material.
Another type of spinal implant may include a cage into which bone growth material is placed. A method of inserting the spinal implant may include forming a disc space that is slightly smaller than a height of the spinal implant and impacting the implant into the opening. Impacting an implant may be dangerous and may not inhibit an inserted implant from backing out of a disc space after insertion. An alternate method of inserting a spinal implant may involve forming a disc space that is slightly larger than a height of the implant to be inserted into the disc space by distracting the vertebrae with a distraction device. After a spinal implant is inserted, the distraction device may be removed. Distracting the adjacent vertebrae a distance that allows a spinal implant to be inserted into a disc space, however, may not be desirable. Imperfect elastic characteristics of connective tissue may not allow connective tissue to return to a pre-distracted state after the distraction device is removed.
Another type of spinal implant includes threading along a substantial portion of a length of the implant. The implant may be screwed into a prepared opening between adjacent vertebrae. The threaded implant may include self-tapping threads, or the spinal implant may be threaded into a tapped opening.